linux-kernel - Re: [PATCH] panic.c: export panic_on

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Message-Id: <1255351683.30919.5.camel@macbook.infradead.org>
Date:	Mon, 12 Oct 2009 13:48:03 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Simon Kagstrom <simon.kagstrom@...insight.net>,
	Artem Bityutskiy <dedekind1@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Koskinen Aaro (Nokia-D/Helsinki)" <aaro.koskinen@...ia.com>,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] panic.c: export panic_on_oops

On Mon, 2009-10-12 at 14:36 +0200, Ingo Molnar wrote:
> Well other than not using sleeping locks in that codepath it should be 
> properly serializable. If the serial driver, netconsole, fbcon and all 
> the other non-trivial console drivers can do it then MTD should be able 
> to do it too.

It's distinctly non-trivial. A 'real' user of the hardware may have just
triggered a block erase, which could take hundreds (or even thousands)
of milliseconds to complete. We can't always suspend that erase; do we
really want to _wait_ for it?

If we're actually in a _panic_ situation, then yes -- we want to wait,
and that's what the device driver's panic_write() method should do.

But for less critical output? I'm not so sure.

Of course, I'm currently looking at revamping the MTD APIs so they're
not entirely synchronous, and are queue-based like the block device API.
Then the console ->write() method could just queue the write, and we
need some other logic to ensure that it really does _happen_ when the
system crashes. A kind of 'purge queue of console writes' function in a
panic handler... except that as you point out, sometimes we actually
want that even when we haven't actually panicked.

-- 
dwmw2

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